An increasing proportion of equipment is becoming digitized, and digital video cassette recorders (hereafter referred to as VCRs) adopting the digital (DV) format have been commercialized.
A digital VCR with DV format of the prior art is explained below according to the document "Specifications of Consumer-Use Digital VCRs using 6.3 mm magnetic tape" published in December, 1994 by HD DIGITAL VCR CONFERENCE.
The DV format can roughly be classified into the standard definition (SD) standard for recording standard television signals and the high definition (HD) standard for recording high definition signals.
FIG. 5 is a block diagram illustrating an example of how conventional digital recording and playback units process video signals in accordance with the SD standard. A shuffling circuit 1 changes the order of video data to average frame data and increase compression efficiency within each frame. A compression circuit 2 compresses the video image, and an error correction code adding circuit 3 adds the error correction signal to the signal compressed by the compression circuit 2. A recording head 17 records the output signal of a digital encoding circuit 4 onto a tape 16. A playback head 18 reproduces the signals recorded on the tape 16, a digital decoding circuit 5 decodes the signals reproduced by the playback head 18, and an error correction circuit 6 corrects data when errors exist. An expansion circuit 7 expands the compressed data to its original size, and a deshuffling circuit 8 reassembles the video data in its original order.
Operation of the conventional digital recording and playback unit as configured above is explained next.
First, the operation by which video data is recorded onto a tape is explained.
The luminance signal constituent of the video signal is sampled and quantized into 8-bit 13.5 MHz form and the color difference signal into 8-bit 6.75 MHz form to generate video data. The shuffling circuit 1 changes the order of the video data, which is recorded data, to average the frame data of the video image for increasing the compression efficiency within the frame. The compression circuit 2 then compresses the shuffled video data to one fifth of its original volume. The error correction code adding circuit 3 adds error correction codes to the compressed video data, and the digital encoding circuit 4 encodes the compressed video data and then sends it to the recording head 17 for recording the data onto the tape 16.
Next, the method by which recorded video data is reproduced from the recorded tape is explained.
The playback head 18 reproduces signals from the tape 16, and the digital decoding circuit 5 decodes these signals. The error correction circuit 6 corrects errors occurring in the data, and the expansion circuit 7 expands the compressed data to its original volume. Finally, the deshuffling circuit 8 reassembles the video data in its original order and the reproducing signals are output.
FIG. 6 shows a format of the tape 16 on which data is recorded. The prior art employs the azimuth recording system using a rotary head. A recording track width is 10 .mu.m and the minimum recording wavelength is 0.49 .mu.m, which is the very high recording density currently used in commercialized VCRs. Data for one frame is recorded onto 10 tracks in the NTSC system, and 12 tracks in the PAL system. The tracks on the tape are roughly divided into four regions from the bottom: the insert track information (hereafter referred to as ITI) region, the audio region, the video region, and the subcode region.
The ITI region holds information for regulating the position of each region when inserting data, on the data structure in each track, and for identifying the track pitch. The audio region holds audio data and its auxiliary information (hereafter referred to as audio AUX), and the video region holds video data and its auxiliary information (hereafter referred to as video AUX). The subcode region typically holds three types of search signals, including the time code which indicates recording hours, absolute track number which indicates the absolute address, and information on recording date and time. Other optional information whose use is designated by the manufacturer may be stored in the subcode region.
The automated track finding system (hereinafter referred to as ATF) is employed for controlling track scanning, and a specified tracking pilot signal is generated in the data when the data is digitally encoded as described above.
FIGS. 7A to 7D illustrate how the ATF system works. Three types of frequency spectrum, as shown in FIGS. 7A to 7C, are available depending on the recording track, and an appropriate frequency spectrum is selected according to the track. The three types of frequency spectrum are called, respectively, track F0 for the type in FIG. 7A, track F1 for FIG. 7B, and track F2 for FIG. 7C. No pilot signal is generated in track F0. The pilot signal, defined as the pilot frequency f1=465 kHz for track F1 and f2=697.5 kHz for track F2, is generated in tracks F1 and F2. Three types of recording tracks are repeated in the order of F0, F1 and F0, F2 as shown in FIG. 7D. During playback, the head detects a component of the pilot signal f1 and f2 leaked from the adjacent tracks F1 and F2 as the head traces the track F0. Tracking control, to assure accurate tracing of recorded tracks, is realized by adjusting the position of the head to maintain the leaked pilot signal at a certain level.
The recording medium has a width of 1/4 inch (6.35 mm). The prior art adopts a two cassette system, consisting of small and standard cassette tapes. The size of the small cassette tape as compared to the existing analog VCR cassette is a mere 1/2 of that of an 8 mm VCR cassette and 1/3 that of a VHS-C cassette. The standard cassette is 1/3.5 of a standard VHS cassette. Standard television signals are recordable for 1 hour on a small cassette and 4.5 hours on a standard cassette. Since the DV format is a new format, it naturally is designed to be smaller than the present analog VCR cassette. However, due to limited capacity, there still exist a series of difficulties in achieving recording hours equivalent to that of analog VCR cassettes, since digitized video data, in spite of being compressed, take up proportionally more recording space than analog data.
To improve the performance of VCR cassettes in terms of recording hours, measures for (1) further improving recording density, (2) increasing the compression rate to reduce information volume, and (3) making the recording medium thinner to increase the recording area per cassette are being studied. In the DV format specifications, the basic direction for realizing longer recording hours has already been determined as additional specifications published in January, 1996.
To improve recording density, the DV format adopts the narrow track system to further narrow the present track width of 10 .mu.m, following the same principle as the super long play (SLP) mode of the VHS system. The narrower recording track width is 6.67 .mu.m, 2/3 of the standard track width of 10 .mu.m.
In a system which records and plays back video images using a common head for standard track and narrow track widths, the common head records and plays back the image without providing a guard band to obtain a complete reproducing signal in the standard track width. In addition, the width of the common head may be required to be wider than the standard track, i.e. wider than 10 .mu.m, to detect the ATF pilot signal from adjacent tracks as mentioned previously.
FIGS. 8A and 8B illustrate the recording and playback operation when a head wider than 10 .mu.m is used for recording and playing back in narrow tracks. The thin solid lines on a recording tape 16 illustrate a recording track pattern for narrow track recording and the bold lines illustrate the scanning trace of the head. The head scans in the order of a, b, c, d, and e. During recording, the head overwrites data with reference to the lower end of the head as shown in FIG. 8A. During playback, the head traces the recorded tracks on its center, maintaining an equal overlapping area with adjacent tracks on both sides, by the aforementioned ATF control as shown in FIG. 8B.
FIGS. 9A and 9B illustrate the recording and playing back operation during assemble recording in narrow tracks. The thin solid lines in FIG. 9A show that the new video image is recorded in narrow tracks over the video image already recorded on the tape in narrow tracks. On starting assemble recording, the digital recording and playback unit of the prior art rewinds the recorded tape for a certain length and operates a short playback to adjust framing to 10 tracks/frame and to obtain information on the previous recording including the time code. At this point the ATF control is applied to the head to trace tracks A, B, and C of the previous recording on its center in the playback mode. On starting new recording, the head overwrites the tape with new recording tracks such as D, E, and F with reference to the lower end of the tracks. On this starting of assemble recording, a part of the last track of the last frame of the previous recording is overwritten by the first track of the new recording as shown by track C in FIG. 9A and causes narrowing of that track as shown by track C' in FIG. 9B. If a narrowed track exists, the level of leaked pilot signal used for ATF control cannot be maintained at a certain level. In other words, the level of the pilot signal leaked from adjacent tracks changes suddenly from track D' of the newly recorded portion when playing back through the previously recorded portion and the new recording portion. This causes uncontrolled tracking for a certain period of time until the level is stabilized. When the ATF system controls the head to trace the track of the old recording on its center, where V is the width of track C' and W is the head width, V is calculated as follows:
V=(6.67.times.3-W)/2 PA1 V: Width of narrowed track (.mu.m) PA1 W: Head width (.mu.m)
If the head width W is at least wider than 10 .mu.m, the width V of track C' becomes narrower than 5 .mu.m. In other words, increased head width further narrows track C', resulting in a greater transition effect between the old recording and the new recording.
FIG. 10 shows video images of assemble recording in narrow tracks displayed when the video image is played back in the normal mode of the conventional digital VCR. A frame image 20 is the image reproduced and displayed, and it is renewed in the sequence of A, B, C, D, E, F, and G as time passes. When deviation in ATF control occurs at the joint portion, video data is not accurately reproduced for a few frames in the new recording after the joint portion as shown in FIG. 10 because of uncorrectable errors occurring in the data for playing back the video image. An image deviating significantly from the ideal may be displayed in frames D and E.
As mentioned above, sudden confusion in ATF control when playing back in the normal mode has a significant and deleterious effect on reproduction of video image data for a few frames in the new recording at a joint portion of assemble recording. The recorded image may not be reproducible and causes interruption in the reproduced image.